ECM Stiffness Sweet Spot (IMAGE) University of Pennsylvania Caption Cancer cells respond to an optimal stiffness of surrounding ECM. If the collagen is too soft, it doesn't pull them hard enough to break cell-cell adhesion and leave the tumor. If it's too stiff, the pores in the ECM are too narrow for the cells to escape. Credit University of Pennsylvania Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.